1. Field of the Invention
The present invention relates to an active distance measuring device suitably used for cameras or the like.
2. Related Background Art
Conventionally, as an active distance measuring device for cameras, a distance measuring device disclosed in Japanese Unexamined Patent Application Publication No. H-10-281756 is known. That is, when an output ratio signal is converted in a CPU into an distance signal corresponding to the distance, in the case where the output ratio signal is at the nearer-side of a clamping effect Y/N determination reference level, which is defined based on the reference object reflectance, the output ratio signal is converted in accordance with a first conversion expression. While, in the case where the output ratio signal is not at the nearer-side of a clamping effect Y/N determination reference level, the output ratio signal is converted in accordance with any one of a first conversion expression and a second conversion expression corresponding to the luminance of the outside light. In the above distance measuring device, when converting the AF signal value into a distance signal value, it is arranged so that, in the case where the AF signal value is at the long-range side more than predetermined threshold value, every signal at the long-range side than that is converted into an infinity signal value corresponding to a predetermined infinity setting value. Owing to this, an object at a long range results in a faint AF signal value; and thus even when the component of noise is not negligible, a precise measurement can be obtained.
Further, in a distance measuring device disclosed in Japanese Unexamined Patent Application Publication No. S-60-189720, it is arranged so that the infinite distance position is determined by level determination means corresponding to the luminance of the object.
[Patent Document 1]    Japanese Unexamined Patent Application Publication (Tokukai) No. H-10-281756
[Patent Document 2]    Japanese Unexamined Patent Application Publication (Tokukai) No. S-60-189720
In the distance measuring device disclosed in Japanese Unexamined Patent Application Publication No. H-10-281756, for example, when the luminance of the outside light is low, since component of noise is small, a precise distance measurement is possible up to a longer distance than the case when the luminance of the outside light is high. However, in the above distance measuring device, since the threshold value is set to a fixed value, the infinite distance determination is made within the short-range side where the AF signal value does not reach a long range limit that is capable of a precise distance measurement. Accordingly, the AF signal value is converted to the infinity signal value corresponding to the predetermined infinity setting value (refer to FIG. 14A). That is, the limit distance (hereinafter, referred to as “reachable distance”), which is capable of obtaining appropriate distance signal corresponding to the distance with respect to the object to be measured, becomes short. FIGS. 14A–14C show graphs for demonstrating the relationship between the distance and the distance signal when the distance measurement is made using the above-described distance measuring device under a condition of 36% object reflectance. FIG. 14A shows a result of distance measurement under a condition of low luminance of the outside light (Lv=7); FIG. 14B shows a result of distance measurement under a condition of medium luminance of the outside light (Lv=14); and FIG. 14C shows a result of distance measurement under a condition of high luminance of the outside light (Lv=16).
Particularly, in the case where the reflectance of the object to be measured is low, since the AF signal becomes faint in the nearer-distance side than that of the case where the reflectance is high, the AF signal is converted to the predetermined signal value in further nearer-distance side (Refer to FIG. 15A). The graph, which represents the relationship between the converted distance signal value and the distance, largely deviates out of an ideal linear graph, and exceeds the line (broken line in the graph), which indicates allowable range of error in the distance measurement to the long range side; thus there may be a case that the error in the distance measurement exceeds the allowable range. FIGS. 15A–15C are graphs, which represent the relationship between the distance and the distance signal when the distance measurement is made under a condition of 9% object reflectance using the above distance measuring device. FIG. 15A shows a result of the distance measurement under a condition of low luminance of the outside light (Lv=7); FIG. 15B shows a result of the distance measurement under a condition of medium luminance of the outside light (Lv=14); and FIG. 15C shows a result of the distance measurement under a condition of high luminance of the outside light (Lv=16). Further, the area enclosed by the two broken lines in FIG. 15 indicates the allowable range of error in the distance measurement.
Also, in a distance measuring device disclosed in Japanese Unexamined Patent Application publication No. 1985-189720, since the setting of the determination level of the infinite long distance is altered in order to cope with the noise of external light, when the luminance of the outside light is high, the influence of the external light becomes large in an area a little closer to the short range, which is determined as infinite long distance, resulting in poor distance measuring property. As a result, the graph exceeds the allowable range of error in distance measurement to the short-range side (refer to FIG. 16C). FIGS. 16A–16C are graphs showing the relationship between the distance and the distance signal when a distance measurement is made using the distance measuring device disclosed in Japanese Unexamined Patent Application Publication No. S-60-189720 under a condition of 36% object reflectance. FIG. 16A shows a result of the distance measurement under a condition of low luminance of the outside light (Lv=7); FIG. 16B shows a result of the distance measurement under a condition of medium luminance of the outside light (Lv=14); and FIG. 16C shows a result of the distance measurement under a condition of high luminance of the outside light (Lv=16). Further, the area enclosed by the two broken lines in FIG. 16 indicates the allowable range of error in the distance measurement.
As described above, in conventional distance measuring devices, there reside such problems that, when the luminance of the outside light is changed, satisfactory reachable distance can not be obtained resulting in poor distance measuring property or the like.